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Evidence showed a digger was in the area of the damaged pipeline about three years ago, which was also confirmed by the landowner's wife and a neighbour.

The council had also used satellite imagery to see if it showed a digger working in the area in the relevant period but regular cloud cover hampered visibility.

No one had been able to confirm who was operating the digger when it struck the pipe but the council was now following a "strong line of inquiry".

It said it was unable to provide more detail to avoid compromising this aspect of the investigation.

The council said the landowner's wife had been co-operative and had also spoken to representatives of NZ Refining and its insurers.

Efforts were now focused on ensuring a methodical, careful and thorough investigation into the cause of the leak.

The council had until March next year to lay any potential charges relating to the spill.

Results of groundwater sampling near the pipeline rupture showed bore water supplies had not been contaminated by the leaking fuel.

The Northland council was working with Refining NZ on repairing the site and clarifying necessary resource consents.

The pipeline rupture, which happened more than a month ago, caused unparalleled disruption to the country's airline industry with scores of international and domestic flights cancelled and thousands of airline travellers stranded here and abroad.

Defence personnel were called in to ferry millions of litres of vital aviation gas to the airport's Wiri terminal while repair work was carried out on the broken section of pipe.